1. Field of the Invention
The present invention relates to a method for producing a polymer composite material in which the polymerization degree of a rigid polymer, dispersed in a flexible polymer and reinforcing it at the molecular level, is increased.
2. Prior Art
Unique properties are generally established in a polymer material due to its high-order structure. The high-order structure is influenced by such a primary structure as molecular weight, molecular weight distribution, crystallinity, orientation and so on, particularly depends largely on its molecular structure. Therefore, there has been macro fiber reinforced composite materials since the past so as to overcome the limit of mechanical properties, which occur naturally.
Examples of filament fibers having a high elastic modulus and a high strength used as a macro fiber reinforced composite material include carbon fibers, glass fibers, alamide fibers and so on. However, these fibers are usually aggregates of fibril and micro fibril and thus contain defects at each place which cause micro cracks. And since the diameter of the filament is about 10 .mu.m, an interface bonding between the fiber and the matrix must be strong enough to distribute the external stress uniformly over oriented fibers in the matrix and draw out a possible ultimate property of the fiber. As for these problems of the structural defects of the macro fiber and the interface bonding, the far-reaching effects of the fatal defects can be reduced and the strength can be enhanced by minuteness of the diameter. That is, the minuteness of the diameter prevents the stress from concentrating at a local part and increase its aspect ratio UD (L and D are the length and the diameter of the reinforced fiber, respectively), thereby increasing the contact area, with the result that the problem of the interface bonding with the matrix molecule can be eliminated. Therefore, it is a best mode to disperse the rigid polymer in the matrix, which mode makes a critical aspect ratio easily achieved if the molecular diameter of the rigid polymer is D, resulting in establishment of the theoretical strength of the molecule if the interface bonding between the fiber and the matrix is enough since rupture of the composite material means rupture of the covalent bond of the molecular chain in the composite material. The rigid molecular chain structure provides the resulting crystal with a molecular chain rigidity due to its molecular structure and also a high strength in the direction of the molecular chain such as the molecular chain can not be folded. A foldable chain structure usually causes a defect at the folded part. Moreover, since a non-crystal part is a propagation path of the destruction crack due to a small number of covalent bonds, the rigid polymer can improves the strength of the composite material.
Therefore, if the molecular chain of the reinforcing polymer has a rigidity over a certain limit and can be dispersed microscopically and uniformly in the matrix polymer, it is expected that a small addition of the reinforcing polymer enhances various kinds of mechanical properties without a decline of the processability.
Hitherto, based on the above idea, concepts of the polymer composite are proposed in Japanese Patent Publication Tokkosho No. 61-5500 and International Patent Domestic Publication No. Sho 55-500440. These prior polymer composites are basically made according to the method in which one polymer and the other polymer are mixed uniformly with the solvent. At the present technical level, it is very difficult to disperse and mix those polymers very uniformly in the matrix, so that we have a problem that many resulting reinforcing materials (agglomerates of the rigid polymer) often have a diameter exceeding a few microns. Even if uniform dispersion can be achieved, the structure is usually thermally unstable and is liable to change into the phase-separation state during molding processing. That is, there are many difficulties in application in industry since the reinforcing material easily becomes rough. Further, it is disadvantageous that we can not use a polymer which is not compatible with a solvent to be used. Moreover, since the use of solvent is regulated by law from various view point at present, it is preferable to provide a non-solvent method.
Then, we provided a polymer composite material including a rigid polymer which is a rod-like reinforcing material having a cross-sectional diameter of 0.07 .mu.m or less when cut at right angles to the longitudinal direction, produced by the method comprising a step of polymerizing the rigid polymer in the flexible polymer matrix with no solvent in the molten and mixed state and a step of dispersing the rigid polymer microscopically at the molecular level (Japanese Unexamined Patent Publication Tokkaihei No. 6-145534 corresponding to U.S. patent application Ser. No. 08/010,710 and European Patent Application No. 93 101 400.5 (Publication No. 0 553 846 A3)).
However, the rigid polymer was found to have a tendency to become rough in the matrix polymer with the mixing and polymerization time. In order to achieve the desired reinforcement effect at the molecular level, polymerization time must be controlled such that said diameter of the dispersed particle should not exceed 0.07 .mu.m. On the other side, some monomers of the rigid polymer do not have rapid polymerization rates in the matrix polymer, and in such a case, the mixing and polymerization for a short time may provide the rigid polymer with inadequate polymerization degrees and insufficient mechanical properties.
Then, it is an object of the present invention to provide a polymer composite material and a method for producing the same, wherein the polymerization degree of the rigid polymer is increased, thereby, the reinforcement effect at the molecular level being increased, without the roughness of the rigid polymer during polymerization in the matrix polymer.
The method was invented in view of the fact that, if the rigid polymer having a slow polymerization rate was complexed with the matrix polymer at a state of low polymerization degree similar to that of an oligomer, the rigid polymer could disperse microscopically in the matrix polymer and the polymerization degree was increased by the subsequent polymerization in a solid phase, resulting in providing a rigid polymer having excellent mechanical properties without roughness of the diameter of the dispersed particle of the rigid polymer. According to the present invention, there is provided a method for producing a polymer composite material characterized in that the polymer composite obtained by a flexible polymer (M) and a rigid oligomer or polymer (R.sub.m).sup.n is polymerized in the solid phase at more than the temperature at which said rigid oligomer or polymer can be polymerized and below glass-transition temperature of the rigid oligomer or polymer so as to increase the polymerization degree of said rigid oligomer or polymer (R.sub.m).sup.n. In the present invention, the polymer composite material before the polyzation in the solid state may be produced by polymerizing the flexible polymer (M) and a monomer (R.sub.m) to form the rigid polymer (R.sub.m).sup.n with no solvent in a molten and mixed state.